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Fuel Efficiency and Slow Driving?
vile8 writes "With the high gas prices and ongoing gas gouging in my hometown many people are trying to find a reasonable way to save gas. One of the things I've noticed is people driving exceptionally slow, 30mph in 45mph zones, etc. So I had to take a quick look and find out if driving slow is helpful in getting better mileage. I know horsepower increases substantially with wind resistance, but with charts like this one from truckandbarter.com it appears mileage is actually about the same between 27mph and 58mph or so. So I'm curious what all the drivers out there with the cool efficiency computers are getting ... of specific interest would be the hemis with MDS; how do those do with the cylinder shutoff mode at different speeds?" Related: are there any practical hypermiling techniques that you've found for people not ready to purchase a new car, nor give up driving generally?
I spent some time researching this matter after a discussion at work started about it.
Something that I had observed in my car was that my fuel economy increased as my speed increased.
At a cruising speed of 85mph, I get 26mpg. at 80mph, I got 24mpg. And at 65, i got about 20mpg. This testing was done along I-10 between Jacksonville and Los Angeles. There's lots of room to set the cruise control. A test usually consisted of fueling up, then a hard acceleration to the testing speed and setting the cruise control to handle maintaining the speed for the next 300 to 350 miles. Individual tests were spot checked (repeated somewhere else on the drive).
In researching this, it wasn't a matter that my car is "faster", stronger, or just plain cooler. It's a function of the drag of the vehicle and the RPMs the engine is turning.
Most cars make their best fuel economy somewhere between 1800 to 2200 rpm. Ah ha! My car has a 6 speed stick. If I'm in 6th gear it's turning about 2000rpm at 85mph.
I then compared ground speed to engine speed ratios of other cars, partly selected because they were owned by people in the discussion, or because they were fairly common cars. Depending on the vehicle, it's best cruise speed could be anywhere between 45mph to 90mph.
Serious? Seriousness is well above my pay grade.
An American Road & Track issue from many years ago (and I'm damned if I can recall which one) had a long article on the results of some fuel economy studies conducted by BMW.
The findings seemed to show that driving style was more important than overall speed.
The tips, in general, were:
- Keep your speed constant; fluctuations up and down are bad.
- Accelerate to your target speed quickly. Spending time slowly accelerating up to it wastes fuel.
- Be in the highest gear feasible for your engine type and road speed.
- 75% throttle for acceleration, conditions permitting.
- Keep your revs low, and change gears often to keep them low. That said, know your torque curve, and use it; if you have a small 4 cylinder, trying to accelerate at 1000 revs is futile.
Using the holy grail of OSes...
There are lots of little things you can do to save on gas. Many center around efficient stopping.
For example, if I see a red light coming up, I'll often ease off the gas and coast in rather than maintaining speed and then braking near the light like most people do. In addition to saving gas on the way to the light, if the light turns green before you stop then you've also saved the gas it would have taken to accelerate back up to speed.
This tactic can be quite entertaining if, for example, an impatient bozo in a SUV comes up behind you while you're coasting, honks, pulls around you and speeds ahead only to stop at the light, and then you smoke him as you coast through the light just as it turns green.
Buy a Scangauge 2. Google it. It costs around $150. Hook it up to your car, and it will show you shitloads of fuel economy information and more.
Air resistance.
Tyre rolling resistance.
Then approximately in order you have;
Air conditioning off.
Engine RPMs constant and at the peak of the torque curve if you can.
Clean your air filter.
Fit iridium spark plugs.
Use a fuel with a cleaning agent every 6 months or so.
And probably illegal;
Chip your car so it runs at the ideal gas/air mixture, not simply one which will allow it to pass the regulations.
Get rid of your catalytic converter.
Deleted
It's counter-intuitive, but relatively rapid acceleration is far better than slow acceleration.
A long, slow acceleration up to a target speed will use more fuel than a rapid acceleration up to a target speed. This is regardless of the vehicle's horsepower or torque.
Downhill coasting I've covered in other posts in this thread. Suffice it to say that brakes are a safety feature that you don't to have missing in an emergency.
Using the holy grail of OSes...
If someone has already said cruise control, I apologize for restating it, but for me it works. The computer in most modern cars can control the speed of the car within a 20th of a mile. (1/20) That is a lot closer than any human can handle. This prevents you from over/under speeding (I.E. You wanting to go 70 MPH but you waiver between 68 and 72). There are other obvious things you can do though: regular car maintenance being the biggest of them all
Postet this before and got heat for the turning off down-hill. I am doing it where possible and I feel like - you can do what you want, not my business.
There is no question that coasting with engine off uses less fuel (zero) but there are other legitimate questions:
Is it necessary? - in many modern cars the fuel is cut off while engine breaking, so in that case you are using zero or thereabouts fuel anyway. On the other hand any savings on fuel (if any) can be easily offset by the extra wear on the brakes. You could also cause major damage to your transmission if you shift into a low gear or even into reverse by accident while moving at high speed.
Is it safe? - No. It's a bad idea to drive in neutral as you cannot quickly accelerate should the need arise, and also increased strain on the brakes makes it more likely that they will fail at the wrong moment.
Is it legal? - Depends where you live, in California and probably some other states it is actually illegal - look up California Vehicle Code 21710
Negative moral value of force outweighs the positive value of good intentions.
The power needed to overcome air resistance does indeed scale with speed cubed. But you're also going faster. So the total energy losses per distance to air resistance scale with speed squared.
If you mod me Overrated, you are admitting that you have no penis.
Air resistance on reasonably aerodynamic objects is actually more like |v|^1.4, not v^2. Air resistance for objects like bricks (or trucks) is roughly v^2.
Oh yes. My car's got one of those displays and my fuel efficiency is about 10 MPG higher than it was when I first got the car, because I pay attention to how my driving affects efficiency.
...air resistance goes with velocity cubed, not squared.
The power increases with the velocity cubed, but since a faster vehicle covers the same distance in less time the actual energy used per mile only increases with the square of the velocity.
Air resistance still goes like v^2, there's just a different coefficient of drag in front of it depending on the aerodynamics.
Stop! Dremel time!
I have an 2001 Sentra and just inst hooked up a gadget I got from Think Geek (ScanTool, I believe its called) that reads the engine computer through the OBDC2 connector. I can verify that taking my foot off the gas does shut off the injectors if the car is in gear and going fast enough. From the ScanTool manual I infer that this behavior is common, but not universal among cars.
The biggest problem I see on the roads are large cars being driven around with only 1 person in them! How "fuel efficient" is that?
About 9 months ago I got sick of paying high prices for petrol and wanted to do my bit for the environment, so I got my motorbike license and purchased a 125cc scooter. I haven't looked back. Now my wife and I only have two vehicles, a small 2.0 litre car and a 125cc scooter. These two vehicles are fine for everything we need them for.
While I admit scooters aren't right for everybody, they are definitely a possibility for the majority of people. When I try and encourage other people to do their bit and buy a scooter they often come back with the following responses...
"They don't work out that much cheaper once you buy everything." - Complete BS, sure there's a little bit involved in the outlay (Scooter AUD$3000, license/training AUD$500, gear AUD$500, rego AUD$350, comp. insurance AUD$200) but once you're up and running these things run on the whiff of an oily rag. Consider it an investment. Right now I'm doing about 150km per week to and from work (and a little bit of running around on the weekends), I'm averaging about 30km per litre! It costs me around AUD$9 per week to fill up, that's with 98RON premium mind you! So you do the sums and see how much it will save you.
"Scooters are dangerous, you could get killed." - Again, complete BS. I was sceptical about how everyone kept saying how dangerous riding a scooter would be. So I did my research and contacted my state's Transport Authority. The stats were even surprising to a sceptic. In the past four years in my state there's been one fatality on a scooter and only three other fatalities on motorcycles less than 250cc. Once you go over 250cc, people seem to become retards and there are a lot more deaths. Mind you, 2/3 of those were with stationary objects i.e. poles, parked cars etc. People forget too that on a scooter you're a much smaller target and much more agile. The main risk I've faced seems to be people coming into my lane without checking blind spots. In all cases I've been able to successfully swerve/break/accelerate out of the way.
"What about riding in the rain, you'll get wet." - No, you don't if you have the right gear. I bought a weatherproof clothing straight up. I wear this gear over my normal business attire with leather shoes and I'm yet to get wet. I've even ridden in hail and I didn't feel a thing!
"You can't carry much stuff on them." - This is the only place where scooters fall down slightly. But still, scooters offer a few storage options. Most come with storage under the seats - big enough for a bag or your helmet. There's usually a bag hook to hang some shopping bags, you can get a top box put on the back for more storage and I also wear a back pack.
"They're too slow and therefore dangerous." - This is true for the 50cc scooters, which I believe are only suitable for inner city riding. But my 125cc will top out at about 100km/h and will happily cruise at about 90km/h. They are also *very* quick off the line.
Did I also mention they're very FUN to ride?
So what's your excuse?
The logic is that the majority of people are going to drive at a certain speed on any given road regardless (the "85th percentile" rule) and the one doofus going significantly slower than this becomes a very unexpected, slow-moving obstacle which requires people to either hit the anchors suddenly, or attempt to swerve around, both of which are clearly unsafe behaviors.
While most cops won't care about this excuse because they want to maintain a ticket quota, many judges will, assuming no other violation and a good attitude, accept the "I was just keeping up with traffic" line as grounds for dismissal or reduction of a citation. There's a reason for this.
I grant you that this study, and some others like it, mention only accidents and do not discuss or even mention fatalities, but the reduction of total accidents when everyone drives at the 85th percentile is a pretty clear fact. If everyone drove slower this probably wouldn't be the case, but since we aren't going to change the rset of humanity's driving patterns, telling people to drive slower than they should is dubious advice.
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Actually drafting is beneficial to both the lead and tail cars. The lead car gets a boost due to the tail car filling in the vacuum caused by their car. I've never heard if this is actually a measurable difference with non-race cars at highway speeds, but it certainly isn't a good idea even if it is.
You actually don't suck efficiency from the car in front of you. That car actually gets a slight boost in efficiency because a second car following close reduces drag-inducing turbulence off the back of the lead car.
Astonishing that I haven't seen anything about that in the comments (though I haven't read all of it yet): make sure to have appropriate tire pressure can make a huge difference as well.
Go with at least the suggested pressure, maybe even a little (up to 0.5 bar) above and you will notice a difference in fuel consumption too.
Don't get over 0.5 bar more than the suggested pressure because it may effect the tire grip in a bad way.
This will affect your fuel efficiency no matter if you drive fast or slow.
You're right, but the principle applies equally to modern EFI systems. Internal combustion engines are more efficient at full load because power output is increased proportionally more than losses are. (Losses are largely dependent on engine speed rather than the amount of torque being produced.) Combustion efficiency itself also benefits at full load from increased dynamic compression ratio, better mixing of fuel with air, and a decreased surface area to mass ratio (better thermodynamic efficiency).
The test conducted by BMW that I'm familiar with used a 528i as the test vehicle (circa 1980). This is a 2.8l straight-six with EFI. They acclerated the vehicle to a set speed, cruised for a bit and then stopped after a set distance. Four variations were mentioned: (listed from most efficient to least efficient according to BMW's results)
throttle 75% open, up-shifts at 2000RPM
throttle 75% open, up-shifts at 5000RPM
throttle 25% open, up-shifts at 2000RPM
throttle 25% open, up-shifts at 5000RPM
So the recommendation is not to accelerate as quickly as possible, but to keep the engine operating near full load at low RPMs. Note that using 75% throttle rather than 100% is specifically to avoid having the fuel injection switch to a richer mixture as it does at WOT.
At this time BMW was working on the 528e. Since the straight-six runs nice and smooth at low RPMs as it is, they designed one that with a shorter camshaft duration and higher intake air speed for greater low-end torque. They also used taller gearing. IIRC they were able to achieve 40mpg at 37mph in top gear. Although this came at the cost of reduced peak power output. Modern tech like variable valve timing/lift and variable length intakes are aimed at getting the best of both worlds.
Having recently ditched our fuel eating Saab 95 estate (automatic) that averaged 22-25mpg, we've now got a Saab 93 Sportwagon diesel (dreadful name, but nice car)
We've been testing it's mpg a lot recently. Town driving it gets 35-40mpg depending on conditions and traffic. This at speed averaging around 30mph. On Motorways at 70mph, consumption is about 40-42mpg.. at 60mph we easily get 51-53mpg.
The economy savings between 70 and 60mph are quite considerable. At £1.20 a litre (for diesel) that makes quite a difference on a long journey.
I always believed that cars optimum fuel economy had gotten better at higher speeds, but that's clearly not the case in my Saab.
it might be a Vauxhall/Opel Vectra underneath, but it's still a nice car to drive!
A couple of years ago jeremy clarkson did just this test. He found the following... Low rpm http://www.autobloggreen.com/2007/12/26/video-london-to-edinburgh-and-back-on-one-tank-of-diesel-in-an/ enjoy!
It's not in the speed you drive, it's in the way you drive. It's in little things like:
- Don't rev the engine above 2500 rpm, change gears faster.
- When a traffic light ahead is red, take your foot from the accelerator. Modern engines use no (that's right: NO) fuel on engine braking.
- Anticipate to the traffic ahead, e.g. give room for somebody so you can slide into his spot.
- Don't start the engine until you are ready to set off.
And this will absolutely NOT make you drive any slower, but I've seen tests where they showed you could save somewhere in the range of 6 to 15% fuel.
Here's a nice link to a Dutch programme called "the new drive":
http://www.hetnieuwerijden.nl/english.html
http://www.rd.com/your-america-inspiring-people-and-stories/ease-on-down-the-road/article55921.html There are a lot of tips on how to save mileage there... some of which are discussed in the comments of this slashdot story.
Correct Horse Battery Staple: 72 bits of entropy. Enter "Correct H" into google. When it generates the phrase, that's
All of your concerns are addressed in Improve MPG: The Factors Affecting Fuel Efficiency which was Slashdotted more than two years ago.
In a nutshell, yes, the horsepower requirements increase with added speed as your engine must fight with increased drag. So driving slower mitigates the energy required to overcome the additional force.
HOWEVER, going too slow and you don't cover enough distance for the amount of gas you use. In an extreme example, idling at 0mph is an asymptotic point on the mpg graph. All cars have different "sweet spots" where they are optimal. Those "sweet spots" are typically dictated by particular RPM ranges and the corresponding speeds by which a chosen gear will drive the car within those RPM ranges.
Some people will say that simply driving in the highest gear at the lowest speed will always produce the best mpg. Not necessarily so (though generally so). As an example, a 2006 Jeep Wrangler turning 35" tires gets exactly the same mpg in 6th gear as it does in 5th gear at 75mph. This is counter-intuitive but one must remember there are more factors at play within the engine computer than simply RPMs. Looking deeper at the situation reveals that while 5th gear requires more RPMs, the ignition timing is advanced nearly 20 degrees on 6th gear plus the higher RPMs are actually in the engine's torque zone meaning it requires less air (and hence less gas based on the stoichiometric ratio) to produce the same amount of power.
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Engine designers attempt to limit this as much as possible with something called a "windage plate", but, for over-the-road cars, it's a bit of a compromise. There are height constraints on the engine, so the oil pan can be only so large, and drivers don't want to be messing with their oil on a regular basis, so efficiency loses out a little bit when they pick a "top" oil level. Try not filling you oil up to that level. Your car will run fine with the level at the "low" marker on your dipstick and you can reduce windage a bit, which should increase mileage.
You'll need to check your oil level on a regular basis, and, of course, there's no advantage in a dry-sump engine - but I don't think any current autos have those.
While I'm on reducing internal resistance, think about using a lower weight oil, that will keep viscosity at high temps. Both Mobil One and Amsoil are good at this.
The notion here is that another point of resistance in your engine is the oil in your main bearings. These are "plain" bearings, which are actually "oil wave" bearings in that the crank actually rests on a hydrostatic wave. You don't need more viscosity for this than the viscosity of the spec'ed oil at max temp. Better to get an oil that starts out pretty close to this viscosity and then stays there.
Oh - and nobody's mentioned cleaning injectors. Yes, they clog, and they don't all clog at the same rate, so you end up with some cylinders running richer than the others. Not good for max power - or efficiency. Cheap partial fix: start using Techron. Complete fix: take 'em out and send to a specialist shop (note: talk to folks about specialist shop about Techron - all the ones I've spoke with use it). If you're driving a sportscar, there'll be higher flowing injectors available. Interesting for this discussion as you can then pick up a used set of stockers for cheap, send [i]those[/i] to the shop and not be without your ride for long.
Also: has nobody actually mentioned tire pressure, and, for that matter, tire design? there are tradeoffs here, too. Lacking a pyrometer, just try running your tires a bit over spec'ed pressure. There used to be a rule about pressure increase from cold to hot, but that was for bias tires and likely doesn't apply any more. If you've got a pyrometer, you want a nice even temp increase across the treads. And you want [i]dry[/i] air. Nitrogen is best. Scuba-air is second best (it's been very dried out). Otherwise, ask whoever owns the compressor if it's been drained recently. Try a body shop or garage where they're using pneumatic tools ,as moisture's bad for the tools. (time was, body shops were best because of the care they had to take for the paint guns, but they're not run off the same compressors any more).
Look at fuel mileage ratings the next time you're replacing a suit of tires - and remember that you're trading off mileage for something else, but for folks seriously considering some of the driving tactics advocated here, this shouldn't matter much.
Ditto aerodynamics. Air dams are easy to install, do actually work, even sub 65MPH. Get a flexible one so when you hit a curb with it when parking it won't matter.
Oh - and you could just buy a higher mileage car. My wife got one of the first US Priuses. She drives it hard and has been getting 50+ MPG for years now.